For the first time, a reliable and successful method of intramedullary nailing was introduced by Kuntscher. To avoid axial and rotational instability, new generations of intramedullary nailing systems were interlocked proximally and distally. Nowadays the interlocking nailing is the method of choice for treating a range of long bone disorders used by trauma and orthopedic surgeons world widely. This method offers a device with excellent stability against the axial and rotational deformation, involves much less surgical exposure and reduces the risk of infection; however, it fails to exert a compression onto the fracture site and fixed bone fragments.
The important role of compression force in normal bone remodeling and bone healing is well known. The absence of adequate weight bearing forces onto bone tissue, such as what happened in bone fixation with an intramedullary nail may lead to inhibition of osteogenesis in fracture site and whole fixed bone fragments (stress shielding effect). Stress shielding effect can results in an increase in osteoporosis rate, fatigue fracture, mal- and non-union susceptibility (1, 2) (3) (4)
Present disclosure introduces the “compression device” as an add-on for interlocking compression nailing systems provides an active inherent compression onto fracture site and whole bone fixed between proximal and distal part of nail prevents osteoporosis due to stress shielding effect, improve bone healing development and emends deficiencies of interlocking compression nailing and similar devices known in the art.
The special interest in compression nailing started in the late 1960s in an effort to combine the advantages of compression osteosynthesis to the intramedullary nailing. One of the initial compression nails, had a tie rod placed within a Kuntscher nail anchored to the distal fracture fragment by cross pinning. The fracture fragments fixation involves sliding a Kuntscher nail over a tension rod and then fixing the clevis of tension rod to distal bone fragment by a transverse screw. The Kuntscher nail is fixed to proximal bone fragment by a metal lip. After compression achievement, a locking sleeve that is fixed by a screw to the tension rod prevents loss of compression.
Huckstep nail is a four-sided intramedullary compression nail of solid titanium alloy, a mix of 6% aluminum and 4% vanadium. Modulus of elasticity of Huckstep nail material is just about half of conventional nails made of stainless steel or chrome cobalt. Although stress shielding effect is much lower in Huckstep nail in comparison with conventional nails, the stiffness and strength of this nail is noticeably lower than former ones (5).
Telescopic locking nails such as U.S. Pat. Nos. 6,569,165 or 7,608,075 were provided with a lower diameter inner nail mounted in a higher diameter outer tube (telescope part) in proximal portion. A penetrating slot in inner nail and a penetrating hole in outer tube were provided. The proximal locking fastener screws go through both the locking slot of inner nail and the locking hole of telescope part. Consequently, the inner nail can move over a limited distance in the telescope part. After proximal and distal locking, the nail prevents axial and rotational displacement but permits cyclic dynamic compression during weight bearing. Compression also can be applied by means of a compression screw pushes the inner nail in the outer tube (6).
The U.S. Pat. No. 4,875,475 was the first to present an axially inserted compression screw as a simple internal compression device. The device comprises of an intramedullary nail adapted to be driven into a hollow bone and an adjusting means. The proximal portion of nail is provided with an internal threaded portion and a transversely penetrating longitudinal slot penetrates through the nail and will be connected to the bone by a fastener screw. An adjusting means comprises of a bolt as adjusting component provided with an external threaded portion screwed directly into the internal threaded portion of the nail. The bolt is adapted to be brought into direct or indirect engagement with the fastener screw transversely penetrating through the longitudinal slot. By screwing the bolt, the fastener screw will be pressed toward the distal end of the nail.
Although the above mentioned inventions' ideas are valuable enough, none of them are manufactured in series nowadays due to various biomechanical or application obstacles. The interlocking compression nailing systems which were introduced as a new generation of nailing systems provide the option of compression achieved by an internal mechanism. Although a wide range of interlocking compression nailing systems were designed and manufactured such as U.S. Pat. Nos. 7,771,428, 7,867,231 and 7,942,876 with regards to different indications and economic considerations, the method of compression exertion were preserved in almost all of them (7-9). Generally, an interlocking compression nailing system consists of a cylindrical intramedullary nail adapted to be driven into a hollow bone comprising a longitudinal hole. Proximal portion of nail is provided with internal threads and a penetrating longitudinal slot. The distal portion of nail is provided with two transverse penetrating circular bores. A compression force onto fracture site can be achieved by using two different methods of dynamic and contact compression. In dynamic method, after nail insertion in reamed cavity of fractured bone, the intramedullary nail is fixed to bone fragments by two fastener screws inserted in distal bores and a fastener screw inserted in proximal end of longitudinal slot. In this way, the proximal bone fragment can freely move along longitudinal axis of nail in the whole length of longitudinal slot while it is immobilized in rotational and axial movements due to inter-locking. Hence, bone fragments are subjected to weight bearing forces of relevant body parts; however, in this method, fracture site instability and bone healing disturbance can be occurred due to moveable fixation of proximal bone fragment to the intramedullary nail. Furthermore, in the unloaded state, weight of the extremity will cause tensile stresses in fracture site that may pushes the bone fragments apart. To prevent macro-movements in fracture site, in contact method, a compression screw which is screwed to the internal threaded portion of longitudinal hole pushes the proximal fastener screw toward fracture site and approximates bone fragments to each other. In this way, movement range of proximal bone fragment was restricted and macro-movements were prevented; however, even contact method did not also provide the ideal and perfect answer to stress shielding avoidance. It should be kept in mind that interlocking compression nailing systems rely on muscular and weight bearing forces to exert the compression force, without any inherent compression force of device itself. Consequently their privileges diminished in patients treated by non-weight-bearing or postponed-weight-bearing protocols. In addition, frequently presence and absence of compression in fracture site during weight bearing may be painful for patients and also may distract the fracture site (10).
A suggested way to provide an active compression in interlocking compression nailing systems is putting force to bend the shaft of proximal fastener screw. In this way, although a bended fastener screw can provide an active compression, there are some disadvantages. First of all, there is a relatively high risk of fastener screw failure by forcing it to bend. Second, although the materials are used in fastener screws have significantly satisfactory stiffness and strength; they do not have sufficient elasticity needed to impose adequate bending force. Thus merely bending of fastener screw may not provide a sufficient compression force onto fracture site. Third, there is not a reliable measurement method to evaluate the compression force exerted onto fastener screw and fracture site that may lead to screw failure due to over tightening or insufficient compression onto fracture site due to inadequate tightening. In addition, it should be taken into account that a wide range of compression forces onto fracture site may lead to a significant difference in patients' outcomes.
Interlocking compression nailing , beyond of its failure to prevent stress shielding effect, was momentous achievement in the field of traumatology and orthopedic and reached a remarkable improvement in patients' outcome because of significant advantages of device in a broad spectrum of indications such as long bone fractures, pseudarthroses, malalignment, arthrodesis, post traumatic problems and reconstructive orthopedic surgeries. Briefly, the main privileges of interlocking compression nailing method in comparison with other alternatives are post-operative early weight-bearing, easy application, more stability, less surgical exposure and a significant reduction in infection risk (11-14) (.
Finally, although some inventions such as the dynamic compression nail described a modified nail designed to achieve axial compression onto fracture site using self-compression locking screws, interlocking compression nailing system seems irreplaceable method due to a broad spectrum of privileges in comparison with other alternative methods (15, 16).